1. Field of the Invention
The present invention relates to a three-axis (XYZ) acceleration sensor and its method of manufacture, more particularly to a simplified method of providing improved impact resistance.
2. Description of the Related Art
Technology for making an acceleration sensor resistant to destruction by impact is described in, for example, Japanese Patent Application Publication No. 2004-198243. The acceleration sensor described therein has a mass attachment section, a peripheral attachment section surrounding the mass attachment section, beams flexibly linking the mass attachment section to the peripheral attachment section, resistive elements disposed on the surfaces of the beams and producing piezoresistive effects, a mass secured to the mass attachment section, a frame securing the peripheral attachment section to its package so that the rest position of the mass is a prescribed distance above the floor of the package, stoppers attached to the peripheral attachment section to restrict the motion of the mass, which is disposed between the stoppers and the floor of the package, and aluminum reinforcements on the stoppers. Under acceleration in a given direction the mass is displaced in the opposite direction, the beams bend, and the electrical resistance of the resistive elements disposed on the surfaces of the beams changes. The magnitude and direction of the acceleration are calculated from these resistance changes.
If the acceleration is directed upward from the floor of the package, the beams sag and the mass moves downward, stopping when it strikes the package floor. If the acceleration is directed downward toward the package floor, the mass moves away from the package floor, stopping when the outer corners of the mass strike the stoppers. The aluminum reinforcements on the stoppers enable the stoppers to withstand the impact of the mass caused by sudden acceleration in the downward direction. A typical requirement is for the acceleration sensor to be able to survive a fall from a height of one and a half meters (1.5 m), which generates an impact force or acceleration equivalent to about six thousand times the acceleration caused by gravity (6000 G).
A problem with this acceleration sensor is that in order to obtain an impact resistance rating of at least 6000 G, the conventional stopper structure requires the deposition of an aluminum reinforcing film at least several tens of micrometers thick. If this film is deposited by one of the processes commonly used in semiconductor fabrication, e.g., a sputtering process, deposition takes much time and the acceleration sensor cannot be manufactured efficiently. Additional mask deposition and patterning steps are also required, making the manufacturing process still more time-consuming, complex, and expensive. A simpler way to reinforce the stoppers is needed.